The kinetics of sulphur(IV) oxidation by ozone in an aqueous solution were studied in various supporting electrolytes (NaClO4, NaCl, NH4ClO4, Na2SO4), using the stopped flow method. The rate data in perchlorate medium (chosen as reference electrolyte) are empirically correlated by the following expression: r=-d3>/dt=k3>+>-1/2 For each supporting electrolyte studied, the rate constant varies linearly with the ionic strength. The value of the rate constant is extrapolated to zero ionic strength (1.65¿104 mol-0.5 L0.5 s-1). The rate constants are shown to be higher mainly when ammonium chloride and sodium sulphate are added to the solution. The effect of temperature is studied between 13 ¿ and 28 ¿C. The reaction rate is unaffected by the trace presence of metal ion (Mn2+, Fe2+, Cu2+, Fe3+, Cr3+). The rate of oxidation of dissolved SO2 by O3, in water droplets under atmospheric conditions (presence of 2O2>=4.31¿10-5 mol L-1 and 3>1.17¿10-9 mol L-1), is calculated as a function of +> and compared to the oxidation by H2O2. At zero ionic strength the ozone reaction becomes faster than hydrogen peroxide reaction above -log +>=4.70. This effect appears at higher H+ concentration when the ionic strength increases (-log +>=3 in 4 mol L-1 NaCl or 2 mol L-1 Na2SO4). ¿ American Geophysical Union 1994